The electronic states of propyne studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations

The VUV absorption and EELS spectra of propyne (CH3CCH) have been reinvesti gated, and reassigned in the light of multi-reference multi-root CI calcula tions of quadruple zeta quality containing both valence and Rydberg type fu nctions. While the Rydberg states were considered as linear, and hence stud ied as vertical excitations, the known bending of valence states in alkynes led to a parallel investigation of the equilibrium structures of the lowes t singlet and triplet valence states of cis and trans type in C-s symmetry, by GVB calculations. Some of the rotamers from these bent states are clear ly saddle-points. The calculated vibration frequencies of the ground state have been compared with experiment. (C) 1999 Elsevier Science B.V. All righ ts reserved.